Data converters such as analog-to-digital converters (ADCs), digital-to-analog converters (DACs), and track-and-holds (T/Hs) can be used in two different ways. The first way is as a “baseband data converter” in which the data converter samples at a sample rate (“Fs′”) that is at least twice as high as the highest frequency contained in an input signal. In order to prevent aliasing, the input frequencies must be restricted to the frequency range spanning DC to Fs/2, or in other words, the input frequencies must be “confined” to the first “Nyquist zone,” where the first Nyquist zone is the frequency range spanning DC to Fs/2, the second Nyquist zone is the frequency range spanning Fs/2 to Fs, the third Nyquist zone is the frequency range spanning Fs to 3Fs/2, and so on. The second way that a data converter may be used is as a “band-pass data converter” in which input frequencies are confined to one of the higher Nyquist zones. Although the input frequencies are technically aliased, a band-pass data converter can provide high fidelity from any Nyquist zone so long as the data converter has sufficient analog bandwidth and it is protected with an anti-aliasing filter, if necessary to prevent signals in other Nyquist zones from aliasing into the Nyquist zone of interest and corrupting the input signal. The following discussion pertains to band-pass data converters.
Spurious free dynamic range (SFDR) is defined as the ratio of the root-mean-square (RMS) value of the maximum signal component at the input of a data converter to the RMS value of the largest spurious component at its output. SFDR is a very important specification for test and measurement instruments that use T/Hs and ADCs to digitize analog signals such as spectrum analyzers, oscilloscopes, and network analyzers, as well as for test and measurement instruments that use DACs to generate analog signals such as vector signal generators and arbitrary waveform generators.
Data converters can impair SFDR by introducing various types of spurious tones. First, data converters can introduce residual tones that are related to system clocks, data patterns, or other signals used in the system. Second, two or more data converters may be interleaved to provide the equivalent of a single data converter having a higher effective sample rate (Fs′), however interleaved data converters also produce undesired spectral components referred to as “interleave images” due to mismatches between the individual data converters. Third, data converters can introduce harmonic and intermodulation distortion products.
There are many techniques to reduce these spurious tones to acceptable levels. In some applications these techniques are acceptable, however in others they are prohibitively expensive or resource intensive.
What is desired is a data converter having reduced spurious tones.